pretty good for a first try, i know the feeling of figuring out the program. From my understanding, lakes usually come from rivers as a general rule, although, many times the rivers can be underground. I think that rivers add a bit of something, that adds to the landscape though. i am still pretty new, and my rivers are ever evolving.
the mountains seem a bit stiff. as a buddy of mine puts it, they need "greeblies" crevices, break off mountains etc.
I like the relief you added, definitely adds something